In the production of mechanical pulp, such as RMP, TMP, CMP, or CTMP, from comminuted cellulosic fibrous material (e.g. wood chips), it is desirable to effectively recover and use the energy content of the process steam which is inherently produced during the refining process. This process steam is produced by the frictional heat which is developed by relative movement of the refiner components. It is of course also desirable to control the working conditions of the refiner so that it operates optimally, while still making maximum effective utilization of the energy content of the process steam.
In a representative prior art heat recovery system associated with a refiner, described in TAPPI June, 1983 issue, at pages 69 through 70, and Pulp & Paper Magazine, June, 1983 issue, at pages 113 through 117, a system is provided in which clean steam produced from the refiner process steam, in a reboiler, is utilized in conjunction with paper machines operating at the mill, or in other desirable manners. In such a prior art system, refining takes place at superatmospheric pressures which are maintained by means of a regulating valve operatively disposed in the process steam line between the refiner and the reboiler, typically between a separator and a scrubber which are disposed in that line. While such a system does allow significant heat recovery, it also has a number of drawbacks associated with it. For instance, because the valve is disposed in the process line between the refiner and the reboiler a pressure reduction, followed by an inherent temperature decrease, takes place. This means that the reboiler operates at a lower temperature than desirable, with the result that the clean steam produced does not have as high as energy content as possible. Further, in such a prior art system the steam volume before the regulating valve may be too small to obtain effective and stable regulation of the refiner working pressure (which is desirable). For example, such a system may be too susceptible to pressure variations in the process steam line.
According to the present invention, a method and apparatus are provided which overcome the drawbacks in the prior art heat recovery system mentioned above. That is according to the present invention, it is possible to obtain significantly more stable process steam pressure control, so that the working pressure on the refiner or refiners may be controlled to result in more effective energy recovery and a more desirable final product. Additionally, the energy content of the clean steam produced in the reboiler is enhanced, making it much more desirable for any end use to which it is put.
The basic features of the inventive method and apparatus which produce the desired end results that are achieved according to the invention, are primarily related to the repositioning of the pressure control valve that is utilized in the prior art heat recovery steam described above. In particular, according to the present invention the pressure regulating valve is removed from the process line between the refiner and the reboiler, and a pressure regulating valve is utilized in the blow-through process steam discharge from the reboiler. This "seemingly" small change has dramatic results. It allows the reboiler to produce clean steam having a higher temperature and/or pressure, effectively having a higher energy content, so that the amount of useful energy that is recovered from the system is increased. Also, the reboiler then effectively acts as a buffer, or damping means, for damping pressure pulsations which may occur in the process steam line. This allows feedback of process steam to the refiner or refiners, to control the working pressure thereof so that the refining operation is more efficient, this too resulting in more effective energy recovery and/or a better quality (more uniform) pulp.
It is the primary object of the present invention to provide for effective heat recovery and efficiency of operation during the production of mechanical pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.